1. Field of the Invention
This invention relates to optical recording media capable of writing and reading information by irradiation of a light beam. More particularly, the invention relates to rewritable phase change optical recording media having functions of erasing and rewriting recorded information and capable of writing information at a high speed and with a high recording density, such as optical disks, optical cards, optical tapes and so forth.
2. Description of the Prior Art
Technique of conventional rewritable optical recording media by utilizing phase change of recording layer is as follows.
Optical recording media have a recording layer consisting of tellurium as its principal component. A focused laser beam pulse is irradiated for a short period of time to a crystalline recording layer at the time of writing to melt at the spot of the recording layer. The fused recording layer is quenched by thermal diffusion and is solidified, thereby providing recording marks in amorphous state.
Reflectance of the recording marks is lower than that in the crystalline state and the recording marks can be read optically as recording signals.
A laser beam is irradiated to the recording mark portions at the time of erasing to heat the recording layer to a temperature below its melting point but above its crystallization temperature so that the recording marks in the amorphous state can be crystallized and returned to the original non-recorded state.
Definite examples of such rewritable phase change optical recording media, there are, as recording layers, recording materials whose thermal stability is improved by adding small amounts of cross linking elements such as Ge and Sb to tellurium or Te-Se alloy, such as Te.sub.81 Ge.sub.15 Sb.sub.2 S.sub.2 (Japanese patent application publication No. 47-26897) and Te.sub.80 Sb.sub.10 Se.sub.10 (Japanese patent application Kokai publication No. 61-145738).
However, the recording layers involve a drawback that since they cause phase separation into tellurium in the crystalline state and tellurium compounds of the cross linking elements at the time of switching from the amorphous state to the crystalline state, the crystallization speed and the speed of the erasing process or of the writing process of the recording information are low.
To compensate for the erasing speed of the recording layer having such a low crystallization speed, there has been proposed a technique which makes the erasing process by use of an exclusive optical head for the erasing process with a light beam elongated in a travelling direction of the medium.
In accordance with such a technique, however, the optical head is complicated in structure and its mass-production is difficult. Even when the erasing light beam is elongated in cross section in order to improve the erasing speed, the technical upper limit of the elongation is up to about 10 .mu.m and in order to accomplish higher speed writing and erasing processes, a high crystallization speed must be attained.
Known recording layers having a high crystallization speed use Sb-Se alloy (Japanese patent application Kokai publication No 60-155495), Sb.sub.2 Te.sub.3 alloy [K. Watanabe et al, J. Appl. Phys. 54, 1256-1260 (1983)] and TeO.sub.x :Sn (Japanese patent application Kokai publication No. 59-185048), and Te-Ge-Sb alloy [Japanese patent application Kokai publications No. 62-209742 and No. 63-225934; N. Yamada et al, Jpn. J. Appl. Phys., 26, Suppl. 26-4, 61-66(1987)], and the like
The optical recording media using Sb-Se alloy as the recording layer involve a problem that noise in the read signals increases with the writing and erasing cycles. The optical recording media using Sb.sub.2 T.sub.3 alloy as the recording layer is not free from the problem of thermal instability of the recording marks because the crystallization temperature from the amorphous state is as low as below 100.degree. C. Recording media utilizing TeO.sub.x :Sn for the recording layer have shown a fair degree of improvement in or relating to the erasing speed, but the improvement is still not sufficient to overcome all of the problems in the prior art. Furthermore, the optical recording media using the Te-Ge-Sb alloy have drawbacks that when the composition is a high speed erasable composition, deformation is liable to occur and openings are produced in the recording layer at the time of writing and its reliability is low, though it has reversibility of writing and erasing. In the high speed erasable composition, the crystallization temperature in the amorphous state is somewhat low, from 123.degree. to 142.degree. C., so that the thermal stability is low and the erasability is not sufficient. Accordingly, it has not been possible to satisfy simultaneously high speed erasability and thermal stability.